The modes for manufacturing lithium ion batteries (“Li-ion batteries”) are presented in many articles and patents, and the work “Advances in Lithium-Ion Batteries” (ed. W. van Schalkwijk and B. Scrosati), published in 2002 (Kluever Academic/Plenum Publishers) provides a good assessment of them. The electrodes of Li-ion batteries can be produced by means of printing techniques (in particular: roll coating, doctor blade, tape casting). These techniques make it possible to produce deposits having a thickness of between 50 and 400 μm. Depending on the thickness of the deposits, their porosities and the size of the active particles, the power and energy of the battery may be modulated. The inks (or pastes) deposited in order to form the electrodes contain particles of active materials, but also binders (organic), carbon powder making it possible to ensure the electrical contact between the particles, and solvents that are evaporated in the electrode drying step. To improve the quality of electrical contacts between the particles and compact the deposits, a calendering step is performed on the electrodes, after which the active particles of the electrodes occupy around 60% of the volume of the deposit, which means that there is generally 40% porosity between the particles. These porosities are then filled with a liquid or gelled electrolyte, capable of comprising solid ion and/or electrical conducting particles.
Other lithium-ion battery architectures have however been developed for electrical energy micro-storage applications. These are thin-film micro-batteries. To satisfy the requirements of miniaturization and temperature stability, these micro-batteries are all-solid, without binders or lithium salt-based electrolytes and equipped with very thin electrodes, on the order of 2 to 5 microns. Such thin-film battery cells have excellent mass and volume energy densities. Indeed, their electrodes are all-solid, without porosities and therefore totally compact. The electrolyte layers deposited on the electrodes consist of highly insulating ceramic or glass-ceramic materials, capable of being deposited in very fine thicknesses without producing risks of short-circuits or excessive self-discharge.
This battery cell architecture, without binders or lithium salt-based electrolytes, which is all-solid and without porosity, makes it possible to maximize the amount of active material per unit of volume, represented by an increase in mass and volume energy densities.
To prevent said cells from being too resistive, the electrodes must remain thin, and their thickness is preferably below 5 microns, or the electrodes must include conductive phases of lithium ions and/or electrons co-deposited with the active material phases. To produce these thin-film electrodes, a number of techniques have been described.
The chemical vapor deposition technique is commonly used to produce thin layers in the field of electronics. This technique, and all of its variants, make it possible to obtain high-quality electrode thin films without porosities. Similarly, physical deposition techniques may be used.
“Thermal spray technology” techniques are more suitable for the production of relatively thick deposits, whereas physical deposition techniques are more suitable for the production of thin films, with thickness below 5 microns. Physical deposition techniques include a number of variants according to the spraying modes. The vaporization of compounds to be deposited can be performed by radiofrequency (RF) excitation, or ion beam assisted deposition (IBAD). Physical deposition techniques make it possible to obtain very high quality deposits containing almost no occasional defects, and make it possible to produce deposits at relatively low temperatures. The other technologies currently available for producing thin films include embodiments based on densification of particle deposits. Among these techniques, sol-gel deposition may be cited. This technique consists in depositing a polymer network on the surface of a substrate, which polymer network is obtained after steps of hydrolysis, polymerization and condensation. The sol-gel transition appears during evaporation of the solvent, which accelerates the reaction processes at the surface. This technique makes it possible to produce compact very thin deposits. Another technique capable of being implemented in order to produce all-solid thin-film deposits consists in depositing material powders forming the electrode in the form of a green ceramic sheet and densifying said deposit by means of a suitable thermomechanical treatment.
These all-solid thin-film battery architectures have numerous advantages over “conventional” Li-ion batteries. The risks of internal short-circuit and thermal runaway are almost eliminated due to the fact that the electrolyte layer no longer contains combustible organic elements, or porosities in which metal salts would be capable of precipitating (more specifically the lithium ions contained in the liquid electrolytes).
Aside from the risk of short-circuit, the performances of conventional batteries, containing aprotic electrolytes with lithium salts, are highly temperature-dependent, so that their use under extreme conditions becomes very difficult if not impossible.
In fact, these batteries have thick electrodes, and the electrolytes impregnated in the porosities of the electrodes help to accelerate the transport of the lithium ions in the thickness of the electrodes, the diffusion of the lithium ions in the solid phases (active particles) being much slower than the transport of lithium ions in the liquid electrolyte.
However, the kinetics of the transport of lithium ions in the electrolytes and the stability thereof is temperature-dependent. An excessively low operating temperature may lead to precipitation of lithium salts in the electrolyte and to an excessive increase in the internal resistance of the battery due to the reduction in ion conduction properties.
For very high temperatures, the organic materials break down rapidly, the organic solvents are capable of evaporating, the passivation layers on the electrolytes may also dissolve in an exothermal method. All of these phenomena lead to the irreversible deterioration of the battery which may lead to cell combustion.
Although they have numerous disadvantages, electrolytes in the form of aprotic liquids containing lithium salts make it possible to assemble stacks of battery cells in order to produce high-capacity batteries. In fact, these liquid electrolytes serve to produce, very simply, an ionic contact of the battery electrodes in order to produce electrochemical cells.
The porous electrodes of the batteries are then arranged in a stack or spiral and the anodes and cathodes are separated by a porous separator. The electrical connections are then produced by connecting the anode collectors with one another and the cathode collectors with one another. The ionic conduction between the anodes and cathodes is then ensured by impregnation of the liquid electrolyte in the porosities of the battery cell (i.e. in the porosities of the electrodes and the separator located between the electrodes).
When all-solid porosity-free electrodes (and/or electrolytes) are used, this contact becomes almost impossible to produce because the mechanical contacts between two solids are not “intimate” enough by comparison with a liquid/solid contact in order to ensure a good transfer of ions at said interface.
In addition, thin-film batteries currently consist of an elementary cell, consequently having a planar structure. They consist of a single cathode/electrolyte/anode stack produced by successive deposition of each of said layers, and cannot be assembled in order to produce an all-solid multilayer cell in the form of a one-piece component.
Only parallel electrical connections of a plurality of independent cells may be produced.